A conventional waveguide-type optical switch fabricated on a LiNbO.sub.3 crystal substrate has the characteristics of low light absorption, low loss, and high efficiency due to its high electro-optical effect.
The conventional waveguide-type optical switch is disclosed, for example, in Shoji Yamada et al., "DC Drift Phenomena in LiNbO.sub.3 Optical Waveguide Devices", Japanese Journal of Applied Physics, Vol. 20, No. 4, April 1981, pp. 733-737.
Referring to FIGS. 1A and 1B, the conventional waveguide-type optical switch is described. FIG. 1A is a perspective view of the conventional waveguide-type optical switch structure and FIG. 1B is a sectional view of the structure.
In the optical switch of FIGS. 1A and 1B, a buffer layer 3 is formed on a LiNbO.sub.3, substrate 1, which includes two Ti-diffused optical waveguides 2a and 2b. Electrodes 4a and 4b of a metallic material are formed on a coupling portion 5 of each of the optical waveguides 2a and 2b via the buffer layer 3, respectively.
The buffer layer 3 prevents light propagating along the optical waveguides 2a and 2b from being absorbed by the electrodes 4a and 4b, etc. SiO.sub.2 is used as the buffer layer 3 because its refraction index of 1.45,. which is smaller than the refractive index of about 2.2 of an LiNbO.sub.3 or LiTaO.sub.3 substrate and because SiO.sub.2 exhibits low light absorption. When the buffer layer's refractive index is small as in the case of SiO.sub.2, it is more feasible to reduce the thickness of the buffer layer 3 to prevent the absorption of light than in the case of a material having a large refractive index.
When switching voltages are applied to the electrodes 4a and 4b, an external electric field concentrates on the buffer layer 3 because the dielectric constant of the buffer layer is smaller than that of the substrate. Therefore, the electric field in the substrate is relatively small. The magnitude of the switching voltages needed for the switching and the modulation becomes higher for larger thicknesses of the buffer layer 3.
Since SiO.sub.2 has a small refractive index and extremely low absorption of light, is no other material for the buffer layer 3 is superior to SiO.sub.2.
However, when SiO.sub.2 is used as the buffer layer 3 and a DC voltage is applied between electrodes 4a and 4b, Li ions, diffused from the substrate into the buffer layer 3 at the fabrication stage, are pulled by the electric field and collected under the electrodes 4a and 4b. Accordingly, an electric field which is counter to the external electric field is generated between the electrodes 4a and 4b. The magnitude of the counter electric field increases as the total moved amount of ions increases with time because SiO.sub.2 has a high ion conductivity despite its relatively high electric insulating property. If the externally applied voltages are kept constant, the resultant electric field applied to the optical waveguides is reduced by the generation of the counter electric field, causing a deterioration of the device characteristics. This shift in an operating voltage point for the switching or modulating operation, known as DC drift becomes a problem when putting the device into practical use.